1. Field of the Invention
The invention relates to a method and apparatus for aerobic composting of organic waste matter in a composting vessel utilizing pressurized air for aeration of the organic matter during the composting process.
2. Discussion of the Prior Art
It is known to compost organic waste material, such as sludge from a waste water treatment facility or garbage, utilizing mechanical composting bins or vessels and pressurized aeration channels. The goal is to produce a nonodorous organic material which can often be sold as a soil additive and which at least is not costly or objectionable to dispose of. In the process, the amount of water in the composting material is reduced, thus resulting in drying of the organic matter to further alleviate any disposal problem.
The usual proposals for mechanical composting have involved upright, cylindrical devices in the nature of silos through which pressurized aeration air is circulated to aid in the decomposition process. Such vertical devices may result in excessive compaction of material which has been vertically stacked. The equipment also tends to be mechanically complicated, with high power consumption.
As an alternative to vertical composting silos, it is known to provide a horizontal composting method and apparatus of a simple construction in which organic matter is moved horizontally along the length of the composting vessel in a stepwise fashion with the use of a hydraulically actuated ram. Diffusers along the length of the vessel inject aeration air into the organic matter to aid in carrying out the composting process in a relatively short period of time. Such a method and apparatus is disclosed in U.S. Pat. Nos. 4,384,877 and 4,436,817 to Nemetz, which U.S. patents are hereby incorporated herein by reference.
Although horizontal, or at least non-vertical, mechanical composters have represented a substantial advance in the art, particularly in that they avoid unduly compacted regions of organic matter in the vessel, while at the same time providing a very simple and economical-to-operate structure, it has been found that even further reduction in the extent of compaction of the organic matter without adding substantially to the complexity or operating cost of the device would be desirable. Such an improvement would avoid the need for cooling of the aeration air injected into the organic matter during composting at times when the surrounding ambient air temperatures are relatively high. In this regard, compaction of the mass of organic matter, which in turn leads to a high pressure drop in the aeration air being injected into the mass of organic matter, requires cooling of aeration air to provide cooling of the mass of organic matter. By achieving a lesser extent of compaction, the amount of energy required for injection of aeration air into the organic matter, which occurs over a relatively long period of time, could also be significantly reduced. Both the temperature and the pressure of the aeration air are interrelated, in that increased pressure results in an increased temperature of the aeration air. During hot weather, the temperature increase over ambient temperature due to pressurization of the aeration air may cause the temperature of the aeration air to exceed the maximum safe level for aerobic decomposition. To counteract this with air cooling equipment is expensive, both in terms of capital investment and operating costs.
By decreasing the extent of compaction, i.e., by decreasing the density of the mass of organic matter being moved through the vessel by the hydraulic ram, the required pressure for effecting penetration of aeration air could be reduced. This, in turn, would save energy in driving the blowers producing the aeration. It would also eliminate the need for expensive cooling equipment at times of relatively high ambient temperatures.
Further, by reducing the amount of compaction of the mass of organic matter resulting from pushing the mass through mass vessel, it becomes practical to build larger composting vessels than would otherwise be practical without the present invention. Normally, as the composting vessel is made larger, the ram pressure needed to push the larger mass through the vessel increases and so does the degree of compaction of the mass. With the present invention, however, the degree of compaction is reduced, permitting construction of a longer vessel which does not cause excessive compaction of the mass. The longer vessel handles higher volumes of organic matter with less capital investment per given unit of compost produced. That is, using the present invention, more compost may be produced by using a longer vessel without increasing the cost of, for example, the infeed conveyor, the ram, and the ram drive mechanisms.